1. Field of the Invention
This invention relates to the detection of corrosion and more particularly to apparatus for measuring and monitoring corrosion of metallic pipe or other metallic conduits through which fluids flow.
Much industrial equipment, such as pipelines, oil refineries and chemical and power plants require the flow of fluids through various types of metallic conduits. Often the fluids are at high temperature and sometimes the fluids are highly corrosive. Replacement of equipment or protection of equipment from corrosion constitutes an important part of the operating costs of such equipment. If the fluids are inflammable or otherwise hazardous, it is important to avoid failure of the conduits to avoid damage to other equipment, danger to operating personnel, and expensive shutdowns. Automatic detection and/or monitoring of corrosion of the conduits helps to avoid failure of the conduits.
2. Description of the Prior Art
A wide variety of corrosion detectors has been developed in an attempt to provide an accurate indication of the corrosion of equipment by fluids with which they are in contact. Such equipment generally includes a test specimen which is suspended in the corrosive fluid. The test specimen most frequently is a thin plate or wire suspended in the fluid whereby the fluid attacks the outer surface of the test specimen and the extent of corrosion is determined by measuring the loss of metal in the test specimen. The simplest and most direct method of measuring the loss of metal is by weighing the test specimen, but that method requires interruption of the corrosion test while the test specimen is withdrawn from the corrosive atmosphere, cleaned and weighed. Another method of measuring the loss of metal caused by corrosion is to measure the increase in the resistance to flow of an electric current through the test specimen. In those devices in which the outer surface of the test specimen is exposed to the corrosive fluid, sealing the specimen without introducing stray galvanic electric currents and protecting the electric leads from the corrosive fluid have been difficult. Moreover, use of thin plates or wires as test specimens does not simulate the conditions to which pipes are exposed in use.
Because the changes in electrical conductivity or resistance of the test specimen are small, accurate measurement of the resistance is essential in a corrosion detecting device. One method that has been used in an attempt to provide accurate measurement has been to compare in a Wheatstone bridge type circuit the electrical resistance of the test specimen with the electrical resistance of a reference specimen that is not exposed to the corrosive fluid. If the test specimen is a ferromagnetic material, such as carbon steel, it is important to avoid unsymmetrical electromagnetic effects. In many of the corrosion detection devices heretofore available, the lack of symmetry of the test specimen and the reference specimen can introduce electromagnetic effects that interfere with or prevent accurate measurements of the resistance. Although some of the electromagnetic effects can be avoided by passing a direct current through the specimens, detrimental thermoelectric effects may be introduced into measurement circuits using a direct current.
Other errors can be introduced into the corrosion monitoring devices of the prior art by failure to maintain the reference specimen and the test specimen at substantially the same temperature. The temperature of the reference specimen should not differ from the temperature of the test specimen by more than 0.5.degree. F. and in no event more than 1.degree. F. If the reference specimen is positioned at some distance from the test specimen or in a different environment, errors in measurement of the resistance of the test specimen may be introduced by difference in temperature between the test specimen and the reference specimen as well as by lack of the necessary symmetry in the system.
Corrosion detecting and monitoring apparatus are disclosed in:
U.S. Pat. No. 2,484,279 by Folz on 10-11-49
U.S. Pat. No. 3,080,747 by Kerst on 3-12-63
U.S. Pat. No. 3,155,933 by Rohrback et al on 11-3-64
U.S. Pat. No. 3,197,698 by Schaschl et al on 7-27-65
U.S. Pat. No. 3,222,920 by Marsh et al on 12-14-65
U.S. Pat. No. 3,320,570 by Lied, Jr. on 5-16-67